The following table is based on the formulse of Reginault, as used by Prof. 

 Guyot, in the preparation of his Psychrometrical Tables, for the Smithsonian Insti- 

 tution viz. : — 



X =f — ^ -, — h, for temperatures above the freezing-point, 



, .480(^ — 7 ^1 

 and X =/ — -V r^ h, for those below : 



XQ which h represents the height of the barometer, t the temperature indicated by 

 the dry bulb centigrade thermometer, t' that indicated by the wet bulb thermometer, 

 / the elastic force of aqueous vapor in a saturated air at the temperature if, and 

 X the actual force at the time of the observation. 



Adapting these formulae to the Fahrenheit thermometer, the former will read 

 _ .480x1(^-0 ,_. .480(^-0 

 ^— •/ 610 — !(«' — 32) -^ IVSO — t' ' 

 and the latter, 



_ .480x1(^-0 7_r -480 (<-0 , 

 ^ --^ 689 — -I (^ - 32) —-^ 1240.2 — tf 

 If we put 7i = 755 millimetres, =29,725 English inches, these formulae may be 

 reduced for the latter measure to the following forms : — 



_ 14.268(^-0 . _. 14.268(^-0 

 ^-•^ 1130-^ ' ^"^''^-Z 1240.2 — ^ • 



In using the table, look out the degree of the wet-bulb thermometer at the top, 

 and the diflFerence between the wet and dry bulb thermometers at the left. Under 

 the former and opposite the latter, find, in their appropriate columns, the force of 

 vapor, and the relative humidity. 



